Charged particle analyzers are useful device components in a number of important analytical tools, including most mass spectrometers and electron spectrometers. These device components provide a means of spatially segregating charged particles on the basis of one or more physical properties, thereby allowing for selective analysis and detection. Dispersive charged particle energy analyzers, for example, are a class of device components wherein a flow of charged particles are exposed to electric and/or magnetic fields that give rise to a spatially varying intensity distribution. Energy dependent deflections caused by the electric and/or magnetic fields segregate the charged particles with respect to position along one or more dispersion axes on the basis of their kinetic energies. Integration of a dispersive energy analyzer with a suitable detector, therefore, allows for measurement of charged particle energy distributions.
In one type of a conventional charged particle analyzer, electrons or other charged particles enter the particle deflector through an entrance aperture and pass between two curved electrodes, for example, see U.S. Pat. Nos. 4,584,474 and 5,357,107. The charged electrodes will cause the charged particles to travel different paths according to the different energies of the particles. The charged particles will exit the deflector through an exit window onto a detector. A field termination system at the exit window is necessary to prevent electric fields generated by the detector and other electronics from distorting the trajectory of the charged particles separated on the basis of their energies.
Electron energy loss spectroscopy (EELS) is an example of such a technique wherein measured electron energy distributions are used to determine physical and chemical properties of bulk solids, interfaces, and gas phase materials. EELS provides a highly sensitive (e.g. <0.1% of a monolayer in some cases), nondestructive and broadly applicable analytical technique, particularly useful for characterizing the composition and physical properties of surface adsorbates. This technique has been successfully applied to the analysis of a range of materials including single crystalline metals, semiconductors and insulators, vapor deposited films, and evaporated materials. Other useful applications of EELS include fundamental studies of molecular surfaces, and catalytic reactions on solid surfaces.
In EELS, a beam of electrons having a selected, essentially monochromatic energy distribution is provided incident to a sample material undergoing analysis. A portion of the incident electrons are inelastically scattered by processes that involve energy transfer to the sample material. The scattered electrons are subsequently collected, and analyzed with respect to energy distribution using a combination of charged particle collection optics, a dispersive charged particle energy analyzer and a detector. Measurements of electron energy distributions of the inelastically scattered electrons provided by this technique can be related to vibrations, surface phonons, and electronic energy transitions of the sample material undergoing analysis with knowledge of the energy distribution of the incident beam of electrons.
Recent advances in EELS instrumentation have resulted in significant enhancements in the resolution and sensitivity accessible using this technique. A new generation of high resolution-electron energy loss spectrometers (HR-EELS) are now capable of accessing an energy resolution less or equal to about a few meV, depending on the scattering properties of the material undergoing analysis. This improvement in resolution makes this technique comparable to complementary IR-spectroscopy techniques, and greatly expands the overall applicability and usefulness of EELS as an analytical tool.
Advances in multichannel instrument designs have also made a significant impact on the performance of EELS instrumentation. In a single channel EELS spectrometer, electrons are passed through a deflector and a portion of the electrons having a relatively narrow distribution of energies are passed through an exit slit and detected. In the single channel configurations, the energy distribution is usually determined by accelerating or decelerating the electrons by a know amount before they enter the analyzer. In a multichannel EELS spectrometer, the exit slit is replaced by an exit window capable of passing spatially separated electrons having a significantly broader distribution of energies. In the multichannel configuration, the energy distribution is determined by simultaneously detecting the spatially separated electrons using a position sensitive detector, such as a multichannel plate detector. Integration of multichannel analysis and detection systems provides a means of measuring broad electron energy distributions at once, thereby resulting in significant improvements in sensitivity and versatility.
Critical to achieving further enhancements in resolution and sensitivity in multichannel EELS instrumentation is the development of dispersive electron energy analyzers having reduced electron optical aberrations. To provide high resolution it is beneficial that a multichannel electron energy analyzer make use of the energy dispersion of the electrons undergoing analysis across the largest extent of the exit plane possible. A field termination system that at least approximately maintains the electric field established in the analyzer is typically necessary to achieve uniform dispersion of charged particles across the dispersion axes of the analyzer. For analyzers employing a conventional cylindrical charged particle deflector comprising two concentrically positioned cylindrical electrodes the electric potential varies as In(r/ro), where r is the radial position of a charged particle undergoing analysis and ro is the average radius of the two cylinder segments comprising first and second electrodes. Termination of this analyzer configuration via equipotential termination electrodes, such as a partially transparent metallic grid electrode, distorts the logarithmic electrical field distribution and, thus, introduces undesirable aberrations in the positions of energy separated electrons exiting the analyzer. Such aberrations undermine the uniformity of the spatial separation of charged particles achieved in the analyzer, resulting in a significant decrease in resolution.
Several approaches have been pursued to address problems associated with field termination in multichannel electron analyzers. One approach, involves termination using a plurality of electrodes held at systematically varying electric potentials. Ho and co-workers, for example, developed a termination system consisting of an array of 22 individually biased metal ribbons positioned in a vertical orientation perpendicular to the gradient of the electric field (P. W. Lorraine, B. D. Thomas and W. Ho, (1992) Rev. Sci. Instrum. vol. 63, page 1652). Biasing of individual electrodes in the array is reported to generate a selectively varying electric field for termination of a cylindrical charged particle deflector. This termination scheme is susceptible to significant drawbacks, however, including generation of a non-uniform electric field that leads to irregular dispersion of electrons undergoing analysis, thereby decreasing resolution. In addition, this termination scheme requires use of a bias resistor network which adds to the overall cost and complexity of the analyzer.
It is clear from the foregoing that there exists a need for charged particle energy analyzers capable of providing high resolution and good sensitivity. Multichannel energy analyzers are needed having a termination system that provides a continuous and selectively varying electric field at the entrance and/or exit planes of the analyzer. Termination device components and methods providing high charged particle transmission and capable of approximating the logarithmic electric field of a cylindrical deflector are needed to provide high resolution electron spectrometers, including HR-EELS spectrometers.